Wikipedia offers few answers. (Off-Topic)

by iconicbanana, C2-H5-OH + NAD, Portland, OR, Monday, May 11, 2015, 20:25 (3490 days ago) @ dogcow
edited by iconicbanana, Monday, May 11, 2015, 20:30

You probably already took a peek at it.

Before the discovery of its magnetic field in 1974, it was thought that because of Mercury′s small size, its core had cooled over the years. There are still difficulties with this dynamo theory, including the fact that Mercury has a slow, 59-day-long rotation that could not have made it possible to generate a magnetic field.

This dynamo is probably weaker than Earth's because it is driven by thermo-compositional convection associated with inner core solidification. The thermal gradient at the core–mantle boundary is subadiabatic, and hence the outer region of the liquid core is stably stratified with the dynamo operating only at depth, where a strong field is generated.[18] Because of the planet's slow rotation, the resulting magnetic field is dominated by small-scale components that fluctuate quickly with time.

The article also indicates that Mercury's magnetosphere is several orders of magnitude weaker than Earth's.